Differential thermal analysis (DTA) is an old and well-known method for the analysis of materials. Basically, the method consists of applying heat simultaneously to a sample material and a reference material. As the sample material goes through various physical and chemical changes, such as crystallization, melting, freezing, oxidation, etc., its temperature is affected by the changes in internal energy. In simple DTA the differences in temperature between the sample and reference are recorded and, from this data, calculations may be made for determining the internal energy changes occurring in the sample. In a more sophisticated instrument, such as disclosed in U.S. Pat. No. 3,263,494 of the present assignee, a feedback circuit utilizes the differential temperature measurement to effect a current differential, in which case a differential power is computed to determine changes in internal energy.
In prior heating and temperature sensing arrangements, such as taught in the aforementioned patent, the sample has been heated by applying electrical energy to a resistive heating element while the sample temperature is sensed by an independent resistive sensing element or by a thermocouple element. One arrangement employs a relatively small furnace adapted to receive a sample container and includes resistive heating and temperature sensing elements positioned in the furnace relative to the sample container. In another form, the temperature sensing element comprises a thermocouple positioned near but generally spaced away from the furnace. These arrangements suffer from one or more defects including heater-sensor thermal lag, electrical leakage, relatively large sample holder capacity, difficulties in furnace fabrication, a large number of electrical leads, and relative complexity and cost of associated circuitry. Generally each sample being tested or compared in an instrument is placed in a cup or crucible having two resistance elements. One element has a heating current passed through it to heat the sample, the heating current being controlled by comparison with a setpoint. The other resistance element, which may be the same type of resistance coil as the first, is utilized for sensing temperature by measurement of changes in resistance with temperature.
U.S. Pat. No. 3,527,912, also of the present assignee, discloses the use of a single element for both heating and sensing. A special circuit alternates between sensing the element voltage and a preset voltage, and feeds back a square wave representing the difference between these voltages to regulate average heating current in the element. This circuit is not adapted to sensing and utilizing temperature difference between elements for two samples.